docs: def fill_subparser(subparser):
      """Sets up a subparser to download the MNIST dataset files.
  
      The following MNIST dataset files are downloaded from Yann LeCun's
      website [LECUN]:
      `train-images-idx3-ubyte.gz`, `train-labels-idx1-ubyte.gz`,
      `t10k-images-idx3-ubyte.gz`, `t10k-labels-idx1-ubyte.gz`.
  
      Parameters
      ----------
      subparser : :class:`argparse.ArgumentParser`
          Subparser handling the `mnist` command.
  
      """
      filenames = ['train-images-idx3-ubyte.gz', 'train-labels-idx1-ubyte.gz',
                   't10k-images-idx3-ubyte.gz', 't10k-labels-idx1-ubyte.gz']
      urls = ['http://yann.lecun.com/exdb/mnist/' + f for f in filenames]
      subparser.set_defaults(urls=urls, filenames=filenames)
      return default_downloader